Multiple orifice disk valves such as disclosed in U.S. Pat. Nos. 3,207,181 and 3,331,396 by Willis are often used on oil or gas wells where very high pressures prevail. Such a valve has two disks in tight face-to-face engagement. Each disk has two holes and the valve is opened or closed by rotating one of the disks relative to the other to move the holes into or out of alignment. Because of the high pressures the frictional resistance to turning of the movable disk can be extremely high. The turning torque for such a valve can exceed ten thousand inch pounds.
Such valves are used for controlling the flow of fluid from oil and gas wells and precise positioning of the valve opening is important. Such a valve moves from a closed position to a fully opened position with 90.degree. of disk rotation. It is therefore important to turn the valve in very small increments for precise flow control.
Such valves can be operated by a pneumatic or hydraulic stepping actuator. In such an actuator a drive shaft is rotated in a plurality of small increments, each increment corresponding to a pulse of pneumatic pressure applied to the actuator. The drive shaft is geared to the valve stem with a large gear ratio so that the angle through which the valve stem rotates is a fraction of the angle through which the drive shaft rotates in each increment. The large gear ratio also assures adequate torque for operating a valve with a relatively small force input at the stepping actuator.
Considerable difficulty can be involved in a stepping actuator for a multiple orifice disk valve operated at high pressure because of the high torque required to turn the movable disk. The magnitude of the torque is such that the valve stem and the fork used for turning the movable disk elastically deform in torsion by many degrees. For example, when it is desired to move a valve from the closed position, several increments in the stepping actuator can be required to apply increasing torque to the valve stem which "winds up" until sufficient torque is applied to move the valve disk. Once the valve stem and other elements of the structure are "wound up" to the extent required to move the disk, each increment in the stepping actuator is reflected by a corresponding fractional increment of disk rotation. The elastic deformation of the valve stem and turning fork is recovered when the direction of rotation is reversed.
The lag between rotation at the stepping actuator and rotation of the disk is not necessarily a severe problem since the valve opening is often determined by measuring the flow of fluid and adjusting the valve until a desired flow rate is achieved. The principal problem involves operation of the stepping actuator because of the elastic deformation of the valve stem and other components of the drive train between the stepping actuator and the movable disk.
A previous stepping actuator employed a pair of coaxial ratchet wheels fixed together for rotation on a common shaft. A drive pawl engaged each ratchet wheel. A pneumatic or hydraulic actuator connected to such a drive pawl strokes the pawl through a selected distance for engaging a tooth on a ratchet wheel and advancing the ratchet wheel. By using two ratchet wheels such a stepping actuator can be driven in either direction.
A problem with such a stepping actuator when the disk turning torque is high, is the wind-up of the drive train between the ratchet and the movable disk. Thus, when the pawl advances and the ratchet rotates, some of the rotation of the ratchet can be elastic deformation of the drive train. When the pawl retracts to catch the next tooth on the ratchet wheel, the entire drive train can unwind or "back drive" causing the ratchet wheel to retreat following the retracting pawl. When the torque to move the disk is high, the elastic deformation can exceed a full tooth spacing on the ratchet wheel. In such a situation if the pawl advances the ratchet one tooth spacing, the ratchet unwinds the same amount as the pawl retracts and there is no net motion of the disk.
Enlarging the tooth spacing is not regarded as a viable solution to the back drive problem since it degrades the ability to precisely control flow through the valve.
One proposed solution to the back drive problem was to provide a Belleville spring loaded friction plate connected to the ratchet shaft. The force required to rotate the shaft one tooth space increment overcomes the friction of the friction plate plus the friction of the movable disk. Back drive is decreased since the elastic torque is applied against the friction plate. Some of the elastic deformation is thus stored in the drive train instead of being released as back drive of the ratchet. To be effective the friction applied by the friction plate must be a substantial portion of the actual torque required to turn the movable disk. Since this friction acts as the ratchet is advanced, it also reduces the net output of the actuator, thus diminishing the effective torque applied to the movable disk.
Further, the coefficient of friction changes with surface wear, lubrication, temperature and other factors, some of which are unknown. Such changes can make the stepping actuator erratic and unpredictable.
Another proposed solution employs a reverse locking clutch such as a sprag type clutch. Although this can permit successful operation of a stepping actuator, it can add several hundred dollars to the cost.
Another proposed solution is to increase the stroke of the drive pawl for the ratchet to substantially more than one tooth spacing. It is desirable to drive a ratchet more than one tooth spacing and less than two tooth spacings to assure that the ratchet advances one increment. If the stroke of the drive pawl is increased beyond two tooth spacings to compensate for the elastic deformation of the drive train there can be circumstances such that the valve disk is moved two increments instead of the desired one. This can make operation of the valve unpredictable and erratic.
It is therefore desirable to prove a reversible stepping actuator with a high torque output that is not subject to back drive due to elastic wind-up of a drive train without extraordinary cost penalties.